Nanostructures such as nanotubes, nanowires and nanobelts are characterized by novel electronic and optical properties intrinsically associated with their load dimensionality and the quantum confinement effect. Such nanostructures have potential applications in nanoelectronics, advanced composites, field emission devices, sensors, probes, optics and optoelectronics.
Silicon nanowires are of great technological importance in microelectronics. Silicon nanowires exhibit significant differences in physical and chemical properties from bulk silicon. Compared to silicon, germanium nanostructures are of particular interest since the exciton Bohr radius of bulk germanium is larger than that of silicon. This results in more prominent quantum confinement effects. Germanium also offers the advantage of lower processing temperatures with easier integration into conventional devices. Germanium also has much higher electron and hole mobility than silicon making germanium the preferred material for electronic devices scaled down to the Sub-100 nm regime.
Several growth methods are known in the art for the synthesis of germanium nanowires. These include laser ablation, thermal evaporation, super critical fluid synthesis, liquid-state synthesis, molecular beam epitaxy and chemical vapor deposition (CVD). CVD has been the most widely employed of these synthesis methods to grow germanium nanowires from suitable catalysts and germanium precursors such as GeH4, Ge2H6, Ge(C5H5)2, C12H12Ge, Ge—GeO2 and Ge—GeI4. In most of the prior art CVD methods as well as the methods employing thermal evaporation of germanium powders, gold nanoparticles have been typically selected as the catalyst for germanium nanowire growth via the vapor-liquid-solid (VLS) mechanism. This is due to the low utectic temperature of the Ge—Au alloy. Other low melting point metals/alloys that have been employed as the catalyst include Al, Cu, Cu—Ni and Fe(Ge).
Untreated germanium nanowires are reported to oxidize upon exposure to ambient conditions. Since germanium oxide coatings possess unfavorable electronic properties such nanowire oxidation degrades the performance of any nanodevice incorporating germanium nanowires. Accordingly, it is necessary to protect any germanium nanowires from oxidation.
One way to control or limit germanium oxidation is by carbon-encapsulation of the carbon-germanium nanowires. This has previously been done using a chemical vapor deposition (CVD) technique. Specifically, a supported gold catalyst was employed to catalyze the growth of the germanium nanowires which were then dispersed on an amorphous carbon film and annealed. This resulted in the encapsulation of the nanowires by well defined, curved graphene sheets. The presence of gold nanoparticles or residual catalysts on the surface of the nanowires is instrumental in initiating graphene sheet formation. Other methods which have been employed for the preparation of germanium/carbon core sheet nanostructures include the arc discharge synthesis of carbon nanotubes in the presence of germanium metal and the deposition of graphitic coatings on preformed germanium nanowires by their treatment with organic vapors at 700 and 900 degrees C.
The carbon arc discharge method, initially used for producing C60 fullerenes, is the most common and perhaps easiest way to produce multi-walled carbon nanotubes (MWCNTs), as it is rather simple. This method creates MWCNTs through arc-vaporization of two carbon rods placed end to end, separated by approximately 1 mm, in an enclosure that is usually filled with inert gas at low pressure. A direct current of 50 to 100 A, driven by a potential difference of approximately 20 V, creates a high temperature discharge between the two electrodes. The discharge vaporizes the surface of one of the carbon electrodes, and forms a small rod-shaped deposit on the other electrode. In order to produce MWCNTs filled with Ge (or indeed another element), a small cavity is bored out in the anode and packed with the desired element.
A disadvantage of the carbon arc discharge method is the fact that is produces a complex mixture of components which requires further purification, i.e., to separate the MWCNTs from the soot and the residual catalytic metals present in the crude product. Indeed, typical MWCNT yields do not exceed 50 weight percent, while single-walled carbon nanotube (SWCNT) yields can reach about 75 percent. This is also apparent for the Ge-filled MWCNTs reported by Dai et al. and by Loiseau et al. Low resolution TEM images of the Ge-filled MWCNTs obtained by Dai et al. clearly show the presence of significant amounts of spherical soot particles intimately mixed with the filled MWCNTs. Similarly, medium resolution TEM images reported by Loiseau et al. for MWCNTs filled with a variety of elements (including Ni, Sm, Dy, S, Sb, Pd, and Co) reveal the presence of amorphous carbon particles mixed with the nanotubes. This presents a clear disadvantage for the production of Ge-filled MWCNTs for microelectronics applications since purification of the crude product would be required.
The present invention is a simple one or two step synthesis method using a combined germanium and carbon source. The nanowires resulting from the present invention are characterized by high thermal stability and high electrical conductivity. In contrast to the prior CVD methods, the current method is catalyst free while the reaction product is clean and pure, incorporating greater than 95 weight percent germanium nanowires encapsulated in MWCNTs and less than 5 weight percent amorphous material.